"This heart belongs again to darkness. All worlds begin in darkness, and all so end. The heart is no different. Darkness sprouts within it. It grows, consumes it. Such is its nature. In the end, every heart must return to the darkness whence it came."

After Xehanort's heart succeeded in obtaining Terra's body in the Keyblade Graveyard, Terra-Xehanort finds himself facing resistance from his host. The first resistance in the form of Terra's mind taking occupancy within the Lingering Will as it creates a giant force field, known as the Will's Cage, to prevent Xehanort from escaping with his body. Though he lost to the Lingering Will, Terra-Xehanort was caught up in the explosion caused by the destruction of the unstable χ-blade, ending up in Radiant Garden as Terra's heart begins to fight back as well with Terra-Xehanort becoming confused and disorientated by the conflicting hearts.

Aqua sacrifices herself to save what is left of Terra.

Found by Aqua as was led to Radiant Garden by Terra's voice, Xehanort was able to reassert control as his vessel battles Aqua with his Keyblade and a Heartless-like symbiote. After the climactic battle, Terra begins to get the upper hand in his internal conflict with Xehanort, forcing Terra-Xehanort to force Terra's heart out with his Keyblade. However, this weakens Terra-Xehanort's mind he falls into the Realm of Darkness manifested behind him by his Guardian. Aqua dives in after him and sacrifices her armor and Keyblade to save Terra. However, as both hearts were still in Terra-Xehanort, realizing that his consumption of Terra would be prolonged, Xehanort reveals that taking the younger Keyblader user's body was only one of many paths he would take as part of a grand design.

At the time, Xehanort's follower Braig brings Ansem the Wise and Dilan to Terra-Xehanort, the now amnesiac Keyblade wielder lies unconscious with no memory at all. Upon Ansem's request for his name, Terra-Xehanort responds that his name is "Xehanort". On Ansem's order, the amnesiac young man is taken back to the castle by Braig while Dilan takes Aqua's armor and Keyblade.

"Master Ansem. Regarding the experiment I presented the other day... With your permission, I'd like to proceed..."

—Xehanort

Braig ponders if Xehanort is Terra.

Following the loss of his memories, now going by the name "Xehanort", Terra-Xehanort's supernatural abilities allowed him to become one of Ansem's apprentices alongside Braig. Though at first thinking the amnesia was an act, Braig intended to help his benefactor by aiding Xehanort in taking advantage of Ansem's research of darkness that lingered within the hearts of people.

Xehanort offered to use himself in the experiments, the first attempt unlocking some of the memories relating to Xehanort as he and Braig manipulated the other apprentices into conducting his own experiments to go deeper into the research. To that end, they abducted countless people and locked them within the depths of the laboratory where they are subjected to the horrible experiments with the purpose of controlling the mind to force it to reject its sense of self. However, this led to the accidental creation of the Heartless, christened by Xehanort, within the Realm of Light.

At this point, Xehanort had taken Ansem the Wise's name as his own and wrote his own report under this stolen name. Xehanort then began to observe the behavior of the Heartless, using a machine to artificially create Emblem Heartless and having them increase their numbers with living things while stepping himself in darkness to control them. Despite Ansem learned of his actions and ordered a halt to the experiments, Xehanort continued and eventually found the heart to Radiant Garden by chance.

That act led to the collapse of the world's boundaries, which was seen in the form of a meteor shower. Soon after, King Mickey arrived on a Gummi Ship he constructed out of materials from the meteors to investigate the worlds due to recent events. It was while making an attempt to convince Ansem to let him test an experiment of his design, refused and ordered to forget everything he learn thus far in his research, that Xehanort relearned the Keyblade and Princesses of Heart. Suspecting Xehanort, feeling they met before, Mickey advised Ansem to check his lab's data. Upon checking, Ansem learned from his Journal that Xehanort continued the research under his name.

Ansem ordered his apprentices to seal off the laboratory, only for Xehanort to eventually banish Ansem to the Realm of Darkness and had both the seal on the laboratory to be broken and that a room that he called the "Chamber of Repose" be constructed. Once done, Xehanort often went in and spoke to Aqua's Keyblade Armor as if its owner in the room in hopes he would regain more memories of his past. Around this time, as the Heartless begin to surge out of Radiant Garden, Xehanort restored the MCP and Sark to its OS.

Xehanort summoning his Keyblade.

Xehanort made the final pages of his report, which outlined his intentions as well as those of the Heartless: To seek Kingdom Hearts. Xehanort decided to use the Heartless' instincts to make his own plans to gain ultimate power, control, and knowledge.

"I'm not Xehanort. My name is Ansem."

—Xehanort

After knocking out Ienzo and Even in the lab, Xehanort summons his Keyblade, then lunges it at Braig. While Braig is falling down, asking if he is truly Master Xehanort, Xehanort states that his name is Ansem before using the weapon on him. While he created Heartless from the apprentices, Xehanort learned that of the Nobodies prior to his own division into two new beings: the Heartless that called himself "Ansem" and the Nobody known as Xemnas. With both components having the memories and personality traits of their original self, "Ansem" and Xemnas act on the goal to obtain Kingdom Hearts in their own way.

While investigating Ansem the Wise's study within Hollow Bastion, Sora, Donald and Goofy find a portrait of Xehanort, believing him to be "Ansem", as it was removed by Tifa to reveal the password to the DTD. After running into King Mickey, he reveals the truth of "Ansem" as a Heartless of an impostor yet could not remember his true name. After the Battle of the 1000 Heartless, Mickey remembers Xehanort's name upon seeing his Nobody Xemnas. At The World That Never Was, Ansem the Wise mentioned Xehanort while insulting Xemnas.

After their revivals, Dilan, Even, Aeleus, Ienzo and Lea search through Radiant Garden's lab for Xehanort (Terra-Xehanort), Braig and Isa. The portrait of Xehanort remained within Ansem the Wise's study.

While traveling throughtout the Sleeping Worlds, Riku pointed out to "Ansem" how Xehanort lost his power to wield a Keyblade upon becoming a Heartless.

Following Master Xehanort's revival at The World That Never Was, the elderly Keyblade Master boasted about making Terra his vessel and claimed that Terra now belonged to him.

Upon his "birth" in Kingdom Hearts Birth by Sleep, Terra-Xehanort appears to retain Master Xehanort's personality, including his knowledge of Darkness, thus being a mere younger incarnation of the elderly Keyblade Master's ruthlessness and cunning. Shortly after Terra-Xehanort is defeated by the Lingering Will, and loses some of his memories, he ends up in Radiant Garden, and begins to question his existence upon meeting Aqua - by strangling her. By the time he is found by Braig, Dilan and Ansem the Wise, he has lost all of his memories entirely—except for the name "Xehanort".

As one of Ansem's apprentices, Xehanort appears to have had a very professional and no-nonsense attitude, shown by his determination in his studies. He appeared to be easily annoyed by Braig and was shown to be equally as interested in the darkness of hearts as his original incarnation. Xehanort appeared frustrated when Ansem forbade him from continuing his studies and experiments, and ultimately disobeyed his master. Ansem the Wise later refers to Xehanort as "foolish" and that any world of his "would be an empire of ignorance".

Terra-Xehanort's appearance is almost identical to Terra's, only with Xehanort's silver hair, darker skin and yellow eyes.

After becoming Ansem's apprentice, he wore a white lab coat with rather loose sleeves that is partway zipped down. The coat also has pockets roughly where his hips would be. He wears black boots, with a white/silver lining along the top and an indentation on the sides. Underneath his coat, he wears a grey vest overtop a white collar shirt, along with a purple ascot around his neck. In addition his eyes have turned brown, his hair is worn long, with two bangs framing his face. His hair also sports three backward-facing spikes, one at the top of his head, and one on either side, just above his ears.

Being a possessed version of Terra, Terra-Xehanort is able to use many of Terra and Master Xehanort's abilities. He wields Master Xehanort's Keyblade, No Name, and uses it to perform techniques like Ars Solum, a fast combo composed of multiple hits. He can use Meteor and Quake as well, and is one of the few bosses that is able to heal with Curaga. When in desperate need of power, Terra-Xehanort gains access to the Dark Impulse Command Style, as well as the Dark Volley and Ultima CannonShotlocks. Terra-Xehanort also appears to possess enhanced strength, able to effortlessly lift Aqua off the ground by the throat with one hand.

Terra-Xehanort can also call upon his "Guardian", a Pureblood Heartless-like symbiote that floats behind him and serves as both his main source of offense and defense. Its great power also makes it capable of slamming opponents with physical blows, possessing targets, and firing dark energy projectiles. While his Heartless Ansem can also use the Guardian; its first chronological appearance is during Terra-Xehanort's battle with Aqua in Kingdom Hearts Birth by Sleep.

After Master Xehanort possesses Terra towards the end of Kingdom Hearts Birth by Sleep, Terra-Xehanort is able to wield Master Xehanort's Keyblade, No Name. The weapon has a spiky shaft, a gleaming eye found near the teeth, which look similar to a battle ax. It is black and silver in coloration, able to lock and unlock hearts, appearing similar to the Oblivion Keyblade. It also contains two demonic blue eyes: one above the handle and the other one on the head, similar to the Soul Eater, Way to the Dawn and Void Gear.

Terra-Xehanort uses the weapon to perform long, powerful combo attacks, as well as use several powerful desperation attacks. After Terra-Xehanort lost his memories, he lost his ability to wield this weapon, but a flashback in Kingdom Hearts 3D: Dream Drop Distance revealed he regained the ability as an amnesiac sometime later, and he likely has regained his memories along with his ability to wield the Keyblade.